This invention relates to a method of sputter etching surfaces. More particularly the invention relates to a method of sputter etching surfaces wherein the reflectance properties of the surfaces may be predetermined.
Photothermal conversion of solar radiation may be a significant energy source in the future if relatively high conversion efficiencies can be obtained. Photothermal coversion is usually accomplished by surfaces which have high absorbance properties and low reflectance properties. Such surfaces should have low thermal emittance in the infrared region. In addition, these surfaces must be chemically, thermally, and optically stable for as long as ten years.
One known method for obtaining a solar absorbing surface is the modification of the surface texture to alter the optical properties. By means of textural irregularities of appropriate geometry and size, the absorbance of the surface can be increased as a consequence of the numerous reflections sustained by the incident radiation without noticeably modifying the thermal emissivity of the material. It is known that a surface can be sputter etched to produce textural irregularities, but heretofore it has not been possible to control the texturing process to produce the desired reflectance profile. Such control would add greatly to the utility of the sputter etching process.